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7.2 Mean parameters of the Intake Duct 121
system. At first, simplified acoustic boundary conditions are imposed at both inlet and outlet
(u’=0). They can be considered as fully reflecting conditions as if they were a rigid wall. In
a second step, the impedance computed by the SNozzle tool for the inlet of the combustion
chamber is used instead of the simple u ′ = 0 condition.
When the simple condition (u’=0) is used, the Helmholtz solver computes an acoustic mode
around 500Hz which is considered a value far from what is observed in experiments. It is well
known that the resonant modes of a system are fully dependent on the acoustic boundary conditions
used [56]. A viewpoint often put forward is that the bad prediction of the resonant
frequency by the Helmholtz solver is probably due to the simplified treatment of acoustics
at both the inlet and the outlet. In the combustion chamber studied, the flow is shocked at
the outlet. In this case, and for low frequencies, it is known that the reflection coefficient can
be considered close to that of a rigid wall. On the other hand, the same cannot be stated for
the inlet. Acoustic properties of the combustion chamber inlet are fully dependent on the upstream
configuration of the combustor. The question that is investigated in this chapter is then
whether a more accurate description of the inlet impedance leads to large changes in the mode
computed by the Helmholtz solver, hopefully in better agreement with the experiments. This
exercise also serves as an illustration of the capability of the SNozzle tool to represent complex
intakes.
Aeronautical engines are complex systems in which the combustion chamber is present just in a
tiny region after the compressor stages. From the point of view of the combustor, the upstream
region is then composed by several compression stages in which acoustics depends on enthalpy
jumps (compressors), the intake acoustic condition (usually related to the atmosphere) and
the geometry of the entire system. Figure 7.1 shows the intake duct of a typical helicopter
combustor.
SNozzle has been developed for computing the acoustic response of a quasi-1D system to an
acoustic perturbation. This numerical tool considers non-negligible Mach numbers as well as
total pressure jumps in the mean flow. SNozzle is then considered appropriate to evaluate the
acoustic impedance at a specific transversal plane of systems, such as of the present aeronautical
air-intake duct. Once the acoustic impedance is evaluated, this acoustic condition can be
imposed in the Helmholtz solver. Resonant frequencies closer to the experimental measurements
are expected. Insignificant changes in the computed mode would support the idea that
the disagreement between the computed and observed mode is not due to errors in the inlet
impedance description.
7.2 Mean parameters of the Intake Duct
SNozzle needs two thermodynamical parameters (temperature and pressure) , the Mach number
at the outlet (see Fig. 7.1) and the section area as a function of the curvilinear axis (Fig. 7.2).
The position and the total pressure jump due to the compressors is also needed in order to